PROJECT SUMMARY/ABSTRACT Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), is a long-term disabling condition with a wide range of symptoms, often triggered by acute infection. The CDC estimates that in 2022, over 3.5 million Americans are living with ME/CFS, and this number continues to increase as many Long COVID patients develop ME/CFS. Profound neurocognitive sequelae are highly prevalent in ME/CFS. The underlying pathophysiologic mechanism that drives neurocognitive impairments in post-infectious chronic conditions remains poorly understood, hindering the development of therapeutic interventions. Studies in chronic viral infections recognize that the trafficking of proinflammatory monocytes in the brain drives disruption of the blood-brain barrier and promotes a neuroinflammatory state. We hypothesize that post-SARS-CoV-2 onset ME/CFS patients and pre- pandemic ME/CFS patients have similar blood-brain barrier (BBB) disruption that is mechanistically linked to circulating proinflammatory analytes (cytokines and chemokines), altered BBB endothelial integrity, and a subtype of proinflammatory peripheral blood mononuclear cells (PBMCs) known as intermediate monocytes. Thus, disruption in BBB integrity promotes persistent neuroinflammation and altered neuronal activity, contributing to neuropsychiatric sequelae in both pre- and post-pandemic ME/CFS patients.Additionally, there is evidence supporting glymphatic system dysfunction in ME/CFS, which further contributes to the perpetuation of the neuroinflammatory state. We propose cross-sectional imaging to assess BBB integrity, with neuropsychiatric assessments and immunophenotyping in 100 post-SARS-CoV-2 onset ME/CFS patients and 100 who have pre- pandemic ME/CFS patients. In addition, we will incorporate 100 participants as a control group of SARS-CoV-2 infected individuals who fully recovered without lingering symptoms from our team NIH-funded similar study in a Long COVID cohort. First, we aim to assess